VI 



MOEPHOLOGY OF THE HEAET 



371 



V 



D 



R 



With increasing growth in length of the cardiac tube this simple 

 curvature becomes converted into a double flexure the heart taking 

 on a S-shape. Of the two curves which make up the S one which 

 has its concave side towards 



the head represents the orig- D 



inal loop, while the other 

 which is convex towards the 

 head has developed in the 

 portion of cardiac tube lying- 

 posterior to the primary loop. 

 Of these two curves the one 

 last mentioned, that which is 

 morphologically posterior, is 

 in an approximately verti- 

 cal plane. The anterior or 

 primary curve on the other 

 hand shows much variation 

 in position in different 

 Vertebrates. While on the 

 whole it still bulges towards 

 the right side, as did the 

 primary loop, the portion of 

 it formed by the originally 

 headward section of the tube 

 comes in many cases to lie 

 ventral to the other limb of 

 the curve. In other cases 

 this, originally anterior, por- 

 tion of the tube lies for a 

 time dorsal to the other, as 

 is the case in Salamandra. 

 The difference will be appre- 

 ciated by comparing the 



relative positions Of C and V F ' G - 177.— A, diagram to illustrate the flexure of 

 • "U , -j n . y -i CM A anr) 1 78 ^ e cara ^ ac tube in the adult Lepidosiren, as seen 



Of all the lower verte- 

 brates in which the peri- 

 cardiac space is still bounded 

 by rigid inextensible walls 

 it is the group of Lung-fishes 

 that shows the heart at 

 the highest level of evolu- 

 tion. And in correlation 

 with this fact we find that 

 in these fishes the kinking 



of the cardiac tube attains its maximum. In a fully developed 

 Lepidosiren (see below, pp. 376-378) the anterior portion of the 

 cardiac tube (the " conus arteriosus ") has developed a further 



from the ventral side. The portion of the diagram 

 above the horizontal line represents the conus : 

 the portion below the horizontal line would 

 represent the rest of the heart on the assumption 

 that this portion of the cardiac tube possesses a 

 similar curvature to that of the conus. Longi- 

 tudinal lines drawn along the tube mark the 

 originally dorsal (D), ventral (V), right (R), and 

 left (£). 



B shows the spiral twisting produced by straightening 

 out a tube possessing the same flexure as the conus 

 portion in diagram A. 



